An image sensor provides a grid of pixels, such as photosensitive diodes or photodiodes, reset transistors, source follower transistors, pinned layer photodiodes, non-pinned layer photodiodes, and/or transfer transistors, for recording an intensity or brightness of light. The pixel responds to the light by accumulating a charge—the more light, the higher the charge. The charge can be processed by another circuit so that a color and brightness can be used for a suitable application, such as a digital camera. Common types of pixel grids include a charge-coupled device (CCD) or complimentary metal oxide semiconductor (CMOS) image sensor.
Backside illuminated sensors are used for sensing a volume of exposed light projected towards a back surface of a substrate. Backside illuminated sensors provide a high fill factor and reduced destructive interference, as compared to front-side illuminated sensors. The pixels are located on a front side of the substrate, and the substrate is thin enough so that light radiation projected towards the backside of the substrate can reach the pixels. Due to the thinned substrate, a supplemental device (e.g., a carrier wafer) is typically attached to the front surface of the substrate on which one or more sensor elements are fabricated. Since such supplemental devices block or prevent ready access to the bond pads of individual sensor elements, the back surface of the substrate is often processed to form an opening providing access to bond pads of the individual sensor elements. It has been observed that bondability to these bond pads is inadequate. Various methods have been introduced to improve bondability by increasing the thickness of the bond pad layers where the bonding occurs; however, such methods are costly, often complicate processing, and/or decrease sensor element performance.
Accordingly, what is needed is a method for manufacturing a backside illuminated sensor device that addresses the above stated issues.